1. Field of the Invention
This invention relates to metal structures resistant to the harmful effects of concurrent oxidation and corrosion by aqueous electrolyte, and which are capable of conduction between the structure surface and an aqueous electrolyte when made anodic in said electrolyte. This invention more specifically relates to electrode structures for use in plates for lead-acid electric storage batteries and as electrodes for use in other electrochemical processes, usually in contact with an aqueous acidic electrolyte. It is a companion invention to an invention described in a concurrently filed application by the same inventors entitled Negative Electrode for Lead-Acid Storage Battery.
2. Definitions
For the purposes of clarity and conformity in the specification and the claims, the following terms will be defined.
The term "anodically passivatable metal" shall mean a metal which, when in contact with an electrolyte and made anodic within an effective potential range, becomes passive to that electrolyte, usually by formation of an oxidic film. For the purposes of the definition in this application, this oxidic film is a non-conductor of electricity.
The term "valve metal" shall mean an anodically passivatable metal, which metal when made cathodic in the electrolyte in which the metal acting as an anode becomes passivated, readily permits passage of current between the metal surface and the electrolyte, and which rapidly or instantaneously passivates when the current is reversed.
The term "matrix metal" includes the primary valve metals niobium, tantalum, titanium, zirconium and hafnium, and alloys containing more than about 85% of these valve metals and exhibiting essentially the same electrochemical valve characteristics as these metals. Matrix metal also includes silicon-containing materials which anodically form silicon dioxide rich surface coatings as well as aluminum and other anodically passivatable metals which may be severely limited with respect to electrolytes with which they can be used.
The terms "filling metal" or "infiltrating metal" shall include lead, and all metals that may be used in electric storage battery grids, such as tin and antimony, and alloys of lead and these metals as well as all other metals, (for example, manganese and ruthenium) which are characterized by having an electrochemically producible oxide which is an electroconductor and which is insoluble in a given aqueous electrolyte. A filling metal can also contain any dopants which may be added to it in order to impart certain desired characteristics. Dopants in lead may be calcium, arsenic, antimony, tin, silver, etc.
The term "oxidized filling metal" shall include oxidized products of the filling metal such as PbO.sub.2, SnO.sub.2, Sb.sub.2 O.sub.5 or mixtures thereof, along with any dopants or possible contaminate from the path walls of matrix metal, such as TiO.sub.2. When formulae such as PbO.sub.2, MnO.sub.2, etc., are used herein especially in regard to electroconductive oxides, it is to be observed that these oxides are not limited to exact stoichiometric compounds but rather include, for example, non-stoichiometric species such as PbO.sub.1.9 which are well known to have enhanced electrical conductivity compared to true stoichiometric compounds.
The term "structural electrode member" shall mean that portion or portions of an electrode which provides mechanical strength to the electrodes as well as a path for electric conduction within the electrode. When this term is used in the context of a battery, it refers to the positive grid and does not include the active material which is in electrically conductive contact with the structural member. However, in the context of other electrochemical processes, for example, electrowinning, this term refers to the whole of the electrode.
The term "composite electrode" shall mean an electrode having a structural electrode member constructed in accordance with the invention disclosed and claimed herein, wherein the structural electrode member includes a composite of a matrix metal and filling metal. The structural electrode member may comprise the entire composite electrode, or only one of several elements.
Any reference in this specification to a particular metal or alloy as the matrix metal, filling metal or coating metal is made by way of example and not for the purpose of limiting the scope of the invention.
3. Description of the Prior Art
In general, conventional lead-acid batteries (Faure type) have positive plates made from battery grids covered with lead dioxide and have negative plates with grids covered with lead. When manufacturing these batteries, positive and negative plates are produced by first pasting the grids with lead oxide and lead salt mixtures, respectively. The pasted grids are then placed in a sulfuric acid solution for electrolytic forming, where the positive pasted material is oxidized to PbO.sub.2 and the negative pasted material is reduced to pure lead sponge.
One mode of lead-acid battery failure results from corrosion of the grid and separation of active material from the plate, especially on the positive plate. This deleterious action results in part, from the oxidation-reduction reactions which take place between the plate and the electrolyte during the many charging and discharging cycles, thereby causing the current carrying capacity of the grid to decrease and its structural strength to be lost. Furthermore, the constant expansion and contraction of the active material during the charge and discharge cycles causes shedding of the active material from the grid.
Ideally, a battery grid is needed in which its corrosion in acid electrolyte during the electrochemical processes is controlled to balance the corrosion requirements for adhesion of active material with the corrosion limits to maintain optimum structural strength and current carrying capacity for the grid. Another consideration is that the grid be light enough to improve the energy density qualities of the battery.
Electrodes used for other electrochemical processes, such as electrowinning, are subjected to the same oxidative corrosion problem as discussed above for battery grids but of course are not usually electrically reversed. The life of electrowinning electrodes, as well as the purity of the cathode product, could be substantially improved if the electrode was constructed of a substance which resisted corrosion from the electrolyte and the electrochemical processes to which it is subjected.
Very few metals are known that will resist corrosion when placed in an acid environment. Three metals, among others, are known to have excellent acid resistance and that behave in a similar manner are titanium, tantalum and niobium. But when these metals are used in electrochemical processes, they behave as valve metals.
Prior attempts have been made to utilize titanium in the grids of lead-acid battery systems. When titanium is used as a positive grid in lead-acid battery systems, its surface becomes oxidized to form titanium dioxide. Unfortunately, this oxide layer is a semi-conductor and will only conduct electricity in one direction through its surface. As a result, a battery with a pure titanium positive grid cannot be charged. When titanium is used as the negative grid in lead-acid battery systems, it corrodes. This fact thereby effectively precludes the use of titantium per se in a negative battery grid. In general, the same problem will be found in the case of positive grids made from tantalum or niobium.
To prevent the formation of titanium oxide at the surface of titanium electrodes that are polarized positively in acid solutions, the prior art has used inert metallic layers on the electrode surface. When constructing a battery, the active material is pasted on the coated titanium grid. Past experience has found that the active material fails to adhere to the coating of inert metal, because there is essentially no chemical bonding between the inert metal layer and the active material.
Other prior art attempts to solve the problem of connecting active material to a titanium electrode structure include one using a three-layer base having a titanium layer on the inside, a relatively inert material layer, and a layer of lead in contact with the active mass or the electrolyte, depending upon its use. The inert material could be in the form of titanium carbide, titanium silicide, or gold or other metals such as nickel. However, structures made in this fashion fail when the outer lead layer becomes oxidized, thereby exposing the middle layer to the electrolyte. The active material then begins to shed.